Debt is the wrong word, it's more like asset depreciation. The software had a cost to create, and it created some value immediately. Over the lifetime of the so

Debt is the wrong word, it's more like asset depreciation. The software had a cost to create, and it created some value immediately. Over the lifetime of the software the maintenance costs will eventually overtake the value that it originally provided. So the question is not how much interest is there on tech debt, but rather what point does the cumulative maintenance of the system overtake the value it created. That's the point that it needs to be replaced. But good luck, replacing a truck that barely works but gets you to the job site is just as tricky as replacing software that doesn't serve any good.